(1) Field of the Invention
The present invention pertains to a centrifugal actuator of an electric motor having an overspeed guard. In particular, the present invention pertains to an overspeed guard for an electric motor centrifugal actuator in which the overspeed guard closely conforms to the peripheral contour of the component parts of the actuator and prevents the component parts from separating from the centrifugal actuator on overspeed of the actuator.
(2) Description of the Related Art
Electric motors having stators with separate start windings and run windings typically employ centrifugal actuators to control the energization of the two windings. The start winding is energized during start up of the motor, or when the speed of the motor falls below a specified operating speed, so as to create a rotating field in the motor stator and to apply sufficient torque to the motor rotor for starting purposes. However, once the motor has accelerated to a desired operating speed, or a pre-determined percentage of the desired operating speed, the rotor is able to follow the alternations of the magnetic field created by the run windings and the start winding is no longer needed. At this point in the motor's operation the centrifugal actuator automatically switches over energization of the start winding to the run winding.
Usually, the start winding is not intended for continuous use and may fail if not de-energized during normal operation of the motor. Conventionally, a switch referred to as a motor starting switch is provided in the motor for energizing the start winding only during start up of the motor and for de-energizing the start winding once the motor has attained its desired operating speed. A centrifugal actuator is typically employed in switching the motor windings between their start winding and run winding. The centrifugal actuator is mounted on the motor shaft for rotation with the shaft, and is responsive to the speed of the motor shaft for switching the windings between the start winding and the run winding, and vice versa.
A typical centrifugal actuator is disclosed in the U.S. Pat. No. 3,609,421, of Hildebrandt, issued Sep. 28, 1971, and assigned to the assignee of the present application and incorporated herein by reference. Basically, the centrifugal actuator includes a main body that is mounted to the electric motor shaft for rotation with the shaft. A sleeve is mounted on the main body for axially reciprocating movement of the sleeve over the main body between first and second positions of the sleeve relative to the shaft. The sleeve has an annular flange that projects radially outwardly from the sleeve. The annular flange is positioned on the shaft adjacent an electric switch. The switch completes a circuit through the start winding and the run winding of the motor in response to the sleeve moving between its respective first and second axially displaced positions on the shaft.
The sleeve and its annular flange are biased toward the first position of the sleeve on the shaft by a pair of springs. The springs extend transversely across opposite sides of the motor shaft and are connected between a pair of levers positioned on opposite sides of the motor shaft. The levers are mounted on the body of the actuator for pivoting movement of the levers relative to the body. Each of the levers are formed as bell cranks having one end connected to the sleeve and the opposite end connected to a weight mounted on the lever. Each of the levers has an intermediate projection that is received in a slot formed in opposite, radially outward portions of the actuator body. The projections engaging in the slots function as the pivot connections of the levers to the body. The pair of springs exert a biasing force on the levers pulling the weighed ends of the levers radially inwardly, and thereby bias the actuator sleeve toward its first position relative to the shaft.
When rotation of the shaft and the centrifugal actuator reaches a pre-determined speed, the centrifugal force exerted on the weighed ends of the levers causes the weighted ends to move radially outwardly against the bias of the pair of springs. This in turn causes the opposite ends of the levers engaging with the actuator sleeve to move the sleeve from its first axial position relative to the shaft to its second axial position relative to the shaft. This movement of the sleeve and its annular flange automatically switches the windings of the stator from energization of the start winding to energization of the run winding of the motor. When the speed of rotation of the shaft falls below the pre-determined speed, the pair of springs pull the weighted ends of the levers radially inwardly, thereby causing the opposite ends of the levers to move the actuator sleeve from its second position relative to the shaft to its first position relative to the shaft. This in turn causes the annular flange of the actuator sleeve to switch the motor windings from energization of the run winding back to energization of the start winding.
Centrifugal actuators of the type described above and disclosed in the above referenced patent are relatively inexpensive to manufacture and, thus are employed on electric motors in many different applications. However, in some applications of electric motors their shafts are at times subjected to increased speeds over that possible by energizing the run winding of the motor, creating an overspeed condition in the centrifugal actuator.
One example of an operative environment in which an overspeed condition of the actuator can occur is in electric motors that drive flexible augers or snakes that are employed in cleaning out pipes, for example cleaning out tree roots from drainage pipes. Augers of this type are basically comprised of a length of flexible cable having a cutting or drilling instrument at one end and the electric motor at the opposite end. The cutting/drilling instrument is pushed through a length of pipe and is rotated by the flexible cable that, in turn, is rotated by operation of the electric motor. Rotation of the cutting/drilling instrument cuts away plant roots that have penetrated the pipe wall and obstruct drainage through the pipe, for example rain water drainage. At times the cutting/drilling instrument will come into contact with a plant root of sufficient size, or other similar obstruction, that it cannot initially cut through completely which stops the rotation of the cutting/drilling instrument in the drain pipe. With the cutting/drilling instrument stopped, the continued rotation of the electric motor shaft causes the flexible cable to wind up in a similar manner to a torsion spring. This will eventually stop the rotation of the motor shaft. When the motor is turned off to withdraw the auger from the pipe, the wound up flexible cable will function as a torsion spring as it unwinds causing the motor shaft to spin in the opposite direction at an increased rate of speed over that of the running speed of the motor. This increased speed of rotation at times causes the centrifugal actuator mounted on the motor shaft to rotate at such a high rate of speed that the levers and springs will fly apart from the actuator body potentially damaging the electric motor and requiring replacement or repair of not only the centrifugal actuator, but also the electric motor.
Attempts at preventing the overspeed condition of the centrifugal actuator from damaging the electric motor have included the development of a plastic cup-shaped guard that is mounted on the motor shaft adjacent the centrifugal actuator with a cylindrical side wall of the guard surrounding the actuator. However, these prior art guards have required more complex tooling and assembly processes which results in higher production costs. What is needed is a lower cost overspeed guard that prevents component parts of the centrifugal actuator from separating from the actuator and potentially damaging the electric motor controlled by the actuator.